In continuously variable power-branched transmissions or building machine transmissions known from practice, hydraulically actuated frictional shifting elements are commonly used, which in the area of a piston chamber are filled with hydraulic fluid and acted upon by appropriate actuation pressure. The extent of filling of a piston chamber of such a shifting element significantly influences the transmission capacity of the shifting element and the torque that can be transmitted by the shifting element concerned at the time. In addition the actuation sequence of a shifting element, starting from an open operating condition of the shifting element, in which the shifting element can transmit essentially no torque and its transmission capacity is essentially zero, in the direction toward an operating condition in which torque can be transmitted by the shifting element operating in a slipping or slip-free mode, affects the closing behavior of the shifting element and therefore also the shifting comfort, which in turn is decisively determined by the torque applied in the area of a drive output of a vehicle or the variation thereof.
Since, as is known, manufacturing tolerances of mass-produced components used in shifting elements and hydraulic lines can sometimes show an undesirable degree of scatter, for each shifting element an individual calibration should be carried out in order to know the particular filling behavior of the shifting elements and to be able to actuate the shifting element so as to achieve an appropriate level of shifting comfort. The parameters that characterize the clutch filling in each case and which are determined by the calibration are stored in the area of a non-volatile memory of a transmission control system and taken into account when carrying out the actuation of a shifting element during every gearshift.
In this context, two parameters in particular are characteristic for the filling behavior of a shifting element. The first parameter is the so-termed rapid filling time, during which a shifting element is acted upon by a so-termed rapid filling pulse in order to fill the shifting element within short operating times. For this, the piston chamber of a shifting element is acted upon by a defined rapid filling pressure during the rapid filling time. The rapid filling phase is followed by a so-termed filling equalization phase, during which the actuation pressure applied in the area of the piston chamber falls from the level of the rapid filling pressure to a level of a filling equalization pressure that represents the second parameter, and remains there for a defined filling equalization time. At the end of the filling equalization phase, ideally the shifting element is in a defined operating condition in which the transmission capacity of the shifting element is essentially equal to zero, and starting from which, an increase of the actuation force of the shifting element results in an immediate increase of the transmission capacity of the shifting element.
From DE 100 51 537 A1 a method for the automated determination of the rapid filling time and the filling equalization pressure of shifting elements is known, whose filling and engagement process can be divided into a rapid filling phase and a filling equalization phase. The rapid filling time and the filling equalization pressure are determined by indirect measurement in a single run.
Disadvantageously, the known method is only partially suitable for changing a shifting element during operation to the defined operating condition in which the transmission capacity of the shifting element is essentially equal to zero and starting from which an increase of an actuating force results in an immediate increases of the transmission capacity. This follows from the fact that the electro-hydraulic regulation train usually provided for actuating a shifting element shows considerable tolerance variations in the area of the electrical components or the current specification, and accordingly a reference value specification for an actuating current of a shifting element, required for adjusting the filling equalization pressure determined, will most probably not correspond to the current value actually required for adjusting the filling equalization pressure determined.
In contrast to transmissions for passenger cars, which are made with hydrodynamic torque converters and respectively associated converter bridging clutches and which, when the converter bridging clutch is open, enable a correspondingly gentle or delayed response of a drive-train built in that way to variations of the torque transmitted in the drive-train, in vehicle drive-trains with continuously variable power-branched transmissions, which are designed correspondingly inflexibly, abrupt torque changes necessarily result in shifting jerks and for that reason an inaccurate actuation of a frictional shifting element that is actuated electro-hydraulically necessarily has an undesirably pronounced adverse effect on the shifting and driving comfort of a building machine.